1. Field of the Invention
This invention relates to methods and systems for controlling autoignition timing of an internal combustion engine operated in a homogeneous-charge compression-ignition mode.
2. Background Information
A conventional gasoline-fueled internal combustion engine employs spark ignition where the fuel and air are premixed and a spark initiates a flame that propagates through the fuel/air mixture in the combustion chamber. The other common type of internal combustion engine employs compression ignition where the fuel and air are purposely kept separate until shortly before top dead center in the engine when the temperature of the air in the combustion chamber is high due to the compression. The fuel then is quickly injected into the combustion chamber as a very fine mist, which partially mixes with the air and autoignites in the combustion chamber. The timing of the fuel injection timing thus controls the autoignition timing. Diesel engines are illustrative of this type of compression ignition engine.
Homogeneous-charge compression-ignition (HCCI) internal combustion engines are known and offer the potential to reduce fuel consumption and NOx emissions. An HCCI engine employs a premixed fuel/air charge to the combustion chamber as in a spark ignition engine, while the charge is ignited by compression ignition as in a diesel engine when the temperature of the air-fuel charge reaches an autoignition temperature in the combustion chamber. HCCI engines typically are provided with a conventional spark plug for each cylinder and relatively low compression ratios, typically close to those of spark ignition (SI) engines, to permit switching of operation of the engine from the HCCI mode at lower engine torques to the SI mode at higher engine torques without engine knocking.
Control of autoignition timing in an HCCI engine is more difficult than in a diesel engine, which controls fuel injection timing to control autoignition timing. In an HCCI engine, the composition and temperature of the fuel/gas mixture in the combustion chamber must be controlled to control autoignition timing.
It has been proposed to control HCCI autoignition timing using what has been called a negative valve overlap strategy that provides internal exhaust gas recirculation in the combustion chamber. Negative valve overlap control strategy involves trapping hot residual burned gas in the cylinder to subsequently mix with fresh air inducted into the combustion chamber. The trapped burned gas raises the temperature of the air-burned gas mixture to promote autoignition. Autoignition timing (delay) is represented by the equation: t=A exp(E/RT), where t is the time it takes for the mixture in the combustion chamber to autoignite, often called the ignition delay, A is an empirical constant, E is an activation energy and is a function of the composition of the mixture, such as type of fuel, fuel/air mixture, amount of residuals, etc., and R is the universal gas constant. Because the equation expresses an exponential relationship, it is evident that temperature of the mixture plays a key role in determining if and importantly when autoignition will occur.
Pursuant to negative valve overlap control strategy, the exhaust valve closes before top dead center (TDC) and the intake valve opens after TDC such that both valves are closed at TDC of the exhaust stroke. Such strategy controls trapping of hot residual burned gas in the combustion chamber to, in turn, control the autoignition timing. FIG. 5 shows a plurality of intake and exhaust valve lift curves versus crank angle for an HCCI engine for purposes of illustrating the negative valve overlap strategy where different negative valve overlaps are shown for use at different engine torques. In particular, for different engine torques, different pairs of intake and exhaust valve lift curves (e.g., curves 1I, 1E; 2I, 2E; 3I, 3E; and so on) are employed in coordination with one another to provide the desired negative overlap for a particular engine torque. That is, intake and exhaust valve lift curves 1I, 1E would be used in coordination for a particular engine torque, different intake and exhaust valve lift curves 2I, 2E would be used in coordination for a different particular engine torque, and so on. The negative valve overlap control strategy is described by Willard et al. in “The knocking syndrome—its cure and its potential”, SAE 982483, 1998.
When engine speed or torque changes, the autoignition timing of the HCCI engine tends to change. For example, at higher torque, autoignition timing tends to advance, resulting in the increase in heat transfer losses, NOx emissions, and combustion noise. Therefore, the engine control system should adjust to move the autoignition timing back to the optimum crank angle. At lower engine torque, autoignition timing tends to be retarded, resulting in an increase of CO emissions and lower combustion efficiency. The engine control system should adjust to move the autoignition timing back to the optimum crank angle.
Moreover, it is desirable to operate the engine with a stoichiometric air-fuel mixture and with a conventional three-way catalyst for after-treatment of exhaust gases. Control of the mass of trapped hot residual burned gas in the cylinder can provide control of autoignition timing during HCCI engine operation. There is a need to also control air-fuel ratio to provide a stoichiometric mixture for engine operation over a wide range of climate and weather conditions without altering the autoignition timing.
However, use of negative valve overlap as a single control variable in HCCI engine control strategy to control both the autoignition timing and the air-fuel ratio at different operating conditions is problematic in that use of a single negative valve overlap variable in the control strategy offers insufficient degrees of freedom to control the air-fuel ratio, in-cylinder gas temperature, and residual fraction of burned gas in the in-cylinder gas in a manner to provide favorable values for all of these parameters at different operating conditions.